1. Field of the Invention
The present invention relates to a speed controller which controls the rotational speed of a rotating body.
2. Description of the Prior Art
Conventionally, for controlling the rotational speed of a rotating body to become a desired value, such a system is known that uses a signal (FG signal) containing rectangular waveform pulses which are shaped from alternating-current (AC) voltage induced in the stator coil of the motor without connecting a speed generator to the rotating body (for example, "brushless DC motor driving system without provision of the position detecting elements" introduced in the "NATIONAL TECHNICAL REPORT", page 614, Vol. 33, No. 5, Oct. 1987). This system can stably control the rotational speed in a simple construction by using the frequency or the repetition period of the FG signal as speed information. For example, this art has been disclosed in the Japanese Patent Publication No. 57-18434 (1982).
The above frequency or period detecting system fully amplifies the AC voltage induced in the stator coil until it becomes a rectangular-waveform signal, and then, the system generates an error output signal considering that a predetermined edge of the rectangular-waveform signal has speed information.
For example, some systems count clock pulses during a period from a leading edge to a next leading edge of the rectangular-waveform signal of the amplified AC voltage, and then, based on the count value, generate as an error output signal a pulse-width modulation signal (the chopper-type driving system), and other systems convert the count value into an analog voltage as an error output signal.
Accordingly, in order to achieve much higher precision rotational speed control, the period of the rectangular-waveform signal must be set more accurately by equalizing the AC voltage to be induced in the stator coil while the motor rotates at a constant speed.
However, when rotating a motor having 12 poles (a pair of 6 poles) in the rotor and 9 coils in the stator by the 3-phase half-wave driving method, the period of the rectangular-waveform signal generated by amplification of AC voltage induced in these stator coils alternately becomes longer and shorter than a normal period every cycle due to uneven precision in the magnetization of the rotor and in the installation of the rotor and the stator. This variation of the period appears in the control system as an external disturbance having a frequency which is one half the frequency of the rectangular-waveform signal, thus eventually degrading the control characteristic. In particular, although the externally-disturbing frequency is in the inertia region of the control system, it invites critical problem to the control system which requires ultra-high precision for the rotation of rotating members like the cylinder motor of the video tape recorder (VTR) for example. Furthermore, since the motors are rarely adjusted during mass-production stage, it is quite difficult for manufacturers to precisely control the period of FG signal covering the total number of the factory-assembled product.